This invention relates to a molding technique for producing a molded product having a predetermined shape by press-molding and, in particular, to a molding apparatus and a molding method which are suitable for press-molding of a glass optical element, such as a lens, a prism, a mirror, and a diffraction grating and which do not require grinding and polishing after press-molding. This invention also relates to a molding method for producing a glass optical element as the molded product.
In the field of the molding technique for producing a molded product, for example, a glass optical element such as a lens, by press-molding, there is known a method of controlling the thickness of the optical element by contact between upper and lower dies of a mold, by contact between the upper or the lower die and another member, or by the use of a positioning member, such as a stopper, for stopping the upper or the lower die when they are spaced at a predetermined distance. For example, the above-mentioned method is disclosed in Japanese Patent application Publications (JP-A) Nos. S61-205630 (Reference 1) and 2000-264652 (Reference 2).
However, if the thickness is controlled by the contact between the dies or between the die and another member, sink marks or the like will be caused to occur when the volume of a glass in the mold is reduced during a cooling/solidifying process. In view of the above, it is proposed in Reference 1 to further press the mold by the use of an elastic member or the like following the shrinkage of the glass during the cooling/solidifying process.
On the other hand, Reference 2 discloses the technique related to a molding apparatus comprising a fixed die (upper die) and a movable die (lower die) faced to each other. The fixed die is fixed by a die holder to the center of a ceiling of a housing defining a molding chamber. In this technique, the molding apparatus comprises mold driving means for driving the movable die, a stopper for stopping the movable die at a predetermined position where the distance between the movable die and the fixed die corresponds to a thickness having a predetermined ratio with respect to a final thickness of the optical element, position detecting means for detecting a stop position of the movable die stopped by the stopper, stopper driving means for changing the position of the stopper between a locking position and an unlocking position where the movable die is inhibited and allowed to move, respectively, and for adjusting the stop position of the movable die with reference to an output of the position detecting means, and a controller for controlling the stopper driving means and the mold driving means.
In the technique described in Reference 1, the mold is pressed by the elastic member. By such pressing using the elastic member, however, it is impossible to control the load. In press-molding, it is generally impossible to achieve high surface accuracy unless a pressing force and a mold position are finely and precisely controlled even after the molded product is pressed to a predetermined thickness. Particularly, for an optical element having a concave meniscus shape or a biconcave shape, surface accuracy is difficult to achieve. Thus, it is an important problem to accurately control the pressing force (the magnitude of the pressing force and the change with time thereof and the mold position during pressing.
In the technique described in Reference 2, it is impossible to prevent variation in thickness due to the displacement of the fixed die resulting from thermal deformation of the housing. In this technique, the position detecting means is disposed within the molding chamber in the vicinity of a mold portion. Since the mold portion is heated to a temperature around a glass transition temperature (for example, around 600° C.), the position detecting means commercially available does not withstand such a high temperature. Therefore, specially-designed position detecting means must be used. The position detecting means may be provided with cooling water circulating means for cooling or temperature control. However, this results in a complicated and large-sized structure of the position detecting means and in an adverse influence upon the temperature distribution within the molding chamber.